A surface light source equipment is used as a backlight for transmission type liquid-crystal display panels, etc. A liquid-crystal display panel transmits or shields light every each pixel to thereby create a picture, but a liquid-crystal display panel itself does not posses the function of emitting a light, so that a surface light source equipment for a backlight is required.
FIG. 1 is a schematic plan view showing a conventional surface light source equipment used as a backlight. The surface light source equipment 21 comprises a light emission part 23 (such light emission part is called a point light source), in which several (preferably, one) light emission elements such as LED (light emitting diode) chips are gathered in one location to be made minute. With the surface light source equipment 21, the light emission part 23 in the form of a point light source is arranged in opposition to a side (a light incident surface 22a) of a light conductive plate 22, which is made of a transparent resin, such as polycarbonate resin, having a high refractive index. A multiplicity of diffusion patterns 24 are arranged discretely on circular arcs, which are centered on the light emission part 23 to be concentric therewith, on an underside of the light conductive plate 22. As shown in FIGS. 2 and 3(a), the respective diffusion patterns 24 are recessed in the form of a triangular groove on the underside of the light conductive plate 22 to extend in circumferential directions of virtual concentric circles centered on the light emission part 23, and normal lines to reflection surfaces 25 of the respective diffusion patterns 24 are directed parallel to directions (this direction is referred to as r axial direction), which connect between the light emission part 23 and the diffusion patterns 24, as viewed in plan view. Also, the diffusion patterns 24 are formed to be gradually increased in pattern density as they are distant from the light emission part 23.
As shown in FIG. 2, with the surface light source equipment 21, when the light emission part 23 is caused to emit light, the light L outgoing from the light emission part 23 enters the light conductive plate 22 from the light incident surface 22a to propagate to a side distant from the light emission part 23 while repeating total reflection on upper and lower surfaces of the light conductive plate 22. When the light L diffusion-reflected by reflection surfaces 25 of the diffusion patterns 24 on the lower surface of the light conductive plate 22 is incident at an incidence angle, which is smaller than a critical angle of total reflection, upon a light outgoing surface 22b on the upper surface of the light conductive plate 22, it outgoes from the light outgoing surface 22b. 
With the surface light source equipment 21, the light L diffusion-reflected by the diffusion patterns 24 is diffused in a plane, which includes the r axial direction and is perpendicular to the light outgoing surface 22b, but not diffused in a direction out of the plane and advances straight as viewed in a direction perpendicular to the light outgoing surface 22b after being reflected by the diffusion patterns 24. Therefore, light outgoing in an optional orientation about the light emission part 23 is not varied in quantity even when diffused by the diffusion patterns 24, and the quantity of light transmitted in the light conductive plate 22 is determined by the quantity of light outgoing from the light emission part 23 in respective orientations. Accordingly, with such surface light source equipment 21, the light outgoing surface 22b can be wholly and uniformly made bright by making a light L having a quantity according to a distance, over which the light conductive plate 22 is transmitted, in respective orientations in the light conductive plate 22 from the light emission part 23. Therefore, by combining the surface light source equipment with a transmission type liquid-crystal display panel, it is possible to manufacture a liquid crystal display, which is excellent in visibility from a wide direction, and besides it is possible to contribute to saving of power consumption in a liquid crystal display.
The diffusion patterns 24 used in the surface light source equipment 21 is designed to be in the form of a triangular prism as shown in FIG. 3(a), and sides 26 positioned on both side ends of the reflection surfaces 25 are designed to be made perpendicular to a back surface of the light conductive plate 22. However, actually, the both sides 26 of the diffusion patterns 24 are gently inclined due to molded sagging generated when the light conductive plate 22 is molded by injection molding or the like.
When the both sides 26 of the diffusion patterns 24 are inclined, a part of the light L made incident upon the diffusion patterns 24 from the light emission part 23 is reflected by the sides 26 of the diffusion patterns 24, and the light L reflected by the sides 26 is scattered in irregular directions as shown in FIG. 3(b) to be increased in directivity, thus damaging the controllability of light, which propagates in the light conductive plate 22. Also, since the light L reflected by the sides 26 is scattered in irregular directions to become a loss light Ls, the cause for a decrease in brightness of the light outgoing surface 22b is brought about.